Cementing compositions containing substantially spherical zeolite

ABSTRACT

A cementitious composition for cementing an oil or gas well and which exhibits when cured, increased flexural strength and a flexural strength to compressive strength ratio between from about 0.29 to about 0.80, contains a hydraulically-active cementitious material, such as Portland cement, and substantially spherical zeolite. Representative zeolites include natrolite, heulandite, analcime, chabazite, stilbite, and clinoptilolite. The weight percent of zeolite in the cement composition is generally less than or equal to 15 percent. In practice, a well bore may be cemented by pumping the activated slurry and pumping it within the well bore to a pre-selected location and allowing it to solidify.

FIELD OF THE INVENTION

The invention relates to cementitious compositions and to methods ofusing such compositions for cementing oil and gas wells.

BACKGROUND OF THE INVENTION

During construction of oil and gas wells, a rotary drill is typicallyused to bore through subterranean formations of the earth to form aborehole. As the rotary drill bores through the earth, a drilling fluid,known in the industry as a “mud,” is circulated through the borehole.Drilling fluids are usually pumped from the surface through the interiorof the drill pipe. By continuously pumping the drilling fluid throughthe drill pipe, the drilling fluid can be circulated out the bottom ofthe drill pipe and back up to the well surface through the annular spacebetween the wall of the well bore and the drill pipe. The hydrostaticpressure created by the column of mud in the hole prevents blowoutswhich would otherwise occur due to the high pressures encountered withinthe well. The drilling fluid is also used to help lubricate and cool thedrill bit and facilitates the removal of cuttings as the borehole isdrilled.

Once the well bore has been drilled, casing is lowered into the wellbore. A cement slurry is then pumped into the casing and a plug offluid, such as drilling mud or water, is then pumped behind the cementslurry in order to force the cement up into the annulus between theexterior of the casing and the borehole. The cement slurry is thenallowed to set and harden to hold the casing in place. Very low cementcompressive strength is required for this purpose; the requiredcompressive strength being dependent on casing and hole diameter.Generally, a compressive strength of 500 psi is sufficient for anycombination of hole/casing for a typical oil well.

The cement also provides zonal isolation of the subsurface formations,helps to prevent sloughing or erosion of the well bore and protects thewell casing from corrosion from fluids which exist within the well. Inthis scenario the important factor is the final permeability of the setcement, which is strictly related to the solid content of the slurry andconsequently to the compressive strength of the set cement. Thus, toprevent fluid movements, the cement should produce a permeability lowerthan 0.05 milliDarcies. To achieve this, the minimum water content inthe slurry is no greater than around 70% by weight of cement, preferablybetween about 37% to about 50%. Such water cement ratios typicallyrender a compressive strength higher than 1000 psi/48 hours and usuallyhigher than 2000 PSI in 48 hours depending on the type of cement, curingtemperature and other additional components of the slurry.

Typically, hydraulically-active cementitious materials, particularlyPortland cements, are used to cement the well casing within the wellbore. Hydraulically-active cementitious materials set and developcompressive strength due to the occurrence of a hydration reaction whichallows them to set or cure under water. The physical properties of theset cement relate to the crystalline structure of thecalcium-silicate-hydrates formed during hydration. For example,conventional Portland cements form an interlocking crystalline networkof for example, tricalcium silicate, dicalcium silicate, tetracalciumaluminum ferrite and calcium hydroxide crystals. These crystalsinterconnect to form an interlocking crystalline structure whichprovides both flexural strength and a degree of resiliency.

Typical cement compositions used in the prior art have a flexuralstrength to compressive strength ratio (FS/CS) of about 0.1 to about0.25. The strength and durability of the crystalline structure dependslargely on the water to cement ratio, porosity of hard set cement to theextent the pores are interconnected, i.e., to what degree permeabilityis developed.

While the development of cement compositions exhibiting higher flexuralstrength are desired, it further is desired to develop cementcompositions which do not display a corresponding increase incompressive strength. The ability to decouple the flexural strength fromcompressive strength has applications where high compressive strengthsare often not desirable due to low well bore stress conditions or wherelow compressive strength cements are unable to withstand high well borestresses, such as deviated wells, gas wells, geothermal wells, steaminjection wells and deep wells.

SUMMARY OF THE INVENTION

The invention relates to a cementitious composition for cementing an oilor gas well. The composition, which exhibits improved physicalproperties, contains a hydraulically-active cementitious material, suchas Portland cement, and substantially spherical zeolite. The improvedcement exhibits increased flexural strength and less brittleness thanneat compositions which do not contain substantially spherical zeolites.

The zeolite is present in the composition in an amount sufficient toincrease the flexural strength of the cementitious composition, whencured. The flexural strength to compressive strength ratio (FS/CS) ofthe cement composition, when cured, is between from about 0.29 to about0.80 and/or is at least 15% higher than the neat cementitious slurry(defined as the identical inventive cementitious composition without thezeolite).

The compressive strength of the cementitious composition, when cured at350° F. for 72 hours, is similar (within test accuracy) to thecompressive strength of the neat cementitious slurry and preferably 15%lower and more preferably more than 25% lower.

The substantially spherical zeolite is substantially hollow. Preferredzeolites include natrolite, heulandite, analcime, chabazite, stilbite,and clinoptilolite. The weight percent of zeolite in the cementcomposition is generally less than or equal to 15 percent.

The invention further provides a method for converting drilling fluidsto a cementitious material which can be used for oil and gas wellcementing operations which has improved flexural strength and decreasedbrittleness. The cementing method consists of solidifying the aqueousdrilling fluid within the borehole of an oil or gas well by adding to itthe cementitious composition containing the hydraulically-activecementitious material, substantially spherical zeolite and water so thatthe slurry is pumpable. The slurry is then activated and pumped withinthe well bore to a pre-selected location and is allowed to solidifywithin the well bore.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In cementing the well bore of an oil or gas well, a pumpable slurry isformed of a hydraulically-active cementitious material and zeolite. Theslurry is then activated and the activated slurry is then pumped intothe well bore. The slurry is then allowed to set up in the well toprovide zonal isolation in the well bore.

Hydraulically-active cementitious materials, suitable for use in thecementitious slurry, include materials with hydraulic properties, suchas hydraulic cement, slag and blends of hydraulic cement and slag(slagment), which are well known in the art. The term “hydraulic cement”refers to any inorganic cement that hardens or sets due to hydration. Asused herein, the term “hydraulically-active” refers to properties of acementitious material that allow the material to set in a manner likehydraulic cement, either with or without additional activation.Hydraulically-active cementitious materials may also have minor amountsof extenders such as bentonite, gilsonite, and cementitious materialsused either without any appreciable sand or aggregate material oradmixed with a granular filling material such as sand, ground limestone,the like. Strength enhancers such as silica powder or silica flour canbe employed as well. Hydraulic cements, for instance, include Portlandcements, aluminous cements, pozzolan cements, fly ash cements, and thelike. Thus, for example, any of the oilwell type cements of the class“A-H” as listed in the API Spec 10, (1st ed., 1982), are suitablehydraulic cements. In addition, the cementitious material may includesilica sand/flour and/or weighing agents including hematite or barite.

Slagment and Portland cement are preferred cementitious materials.However, both of these materials react quickly with water and set atroom temperature unless modified, and they are, therefore, much moredifficult to control. The interstitial water of both cement and slagmentslurries is also very aggressive, for example, having a high pH. Yet,storable slurries formed from hydraulic cement, especially Portlandcement, or slagment have the best overall performance characteristicsfor well cementing applications.

Zeolites, a group of hydrous aluminosilicate minerals containing sodium,calcium, barium, strontium and potassium, for use in the invention arespherical or substantially spherical and are generally hollow. Suchzeolites, upon introduction of water, can swell and are compressible. Inlight of their elasticity, flexural strength and tensile strength of thehardened cement is increased.

The zeolites for use in the invention are characterized by a rigidthree-dimensional crystalline structure, akin to a honeycomb, consistingof a network of interconnected tunnels and cages. As such, the zeolitesfor use in the invention are hollow. As water is absorbed, it movesfreely in and out of the pores, leaving the zeolite framework rigid.Further, the pore and channel sizes of zeolites are nearly uniform,thereby allowing the crystalline structure to act as a molecular sieve.Such zeolites are substantially spherical and further are substantiallyhollow.

The porous zeolite is host to water molecules and ions of potassium andcalcium, as well as a variety of other positively charged ions. However,only those ions of appropriate molecular size which are capable offitting into the pores are admitted. Thus, the zeolite serves to filterunwanted ions. Further, the zeolites are capable of exchanging cations,i.e., the trading of one charged ion for another on the crystal, and arecharacterized by a high cation exchange capacity (CEC). Zeolites havehigh CEC's, arising during the formation of the zeolite from thesubstitution of an aluminum ion for a silicon ion in a portion of thesilicate framework (tetrahedral units that make up the zeolite crystal).

Thus, unlike the flakes, fibers and lamellar type of materialspreviously used in the air, the spherical zeolites make the cement, whenhardened, less brittle, thereby decreasing compressive strength andincreasing flexural strength and tensile strength. Further, use of theat least substantially spherical zeolites facilitates the mixing of thecement with water at equivalent solid/liquid ratios used with theflakes, fibers and lamellar materials of the prior art. The zeolitesemployed in the invention further typically have no chemicals adsorbedonto the surface.

Preferred zeolites include natrolite (Na₂Al₂Si₈O₁₀.2 H₂O), heulandite(Na, Ca)₄₋₆Al₆(Al, Si)₄Si₂₆O₇₂.24 H₂O), analcime (AlSi₂O₆.2 H₂O),chabazite (CaAl₂Si₄O₁₂.6 H₂O), and stilbite (Ca₂NaAl₅Si₁₃O₃₆.14 H₂O),clinoptilolite (AlSi₅O₁₂.6 H₂O). Especially preferred is clinoptiloliteClinoptilolite, having a silica to alumina ratio of 5 to 1, andchabazite, having a silica to alumina ratio of 2 to 1, are especiallypreferred. In such zeolites, the net negative charge within thesymmetrical voids hold the cations for the cation exchange capacity(CEC). Ion exchangeable ions, such as potassium, calcium, magnesium andsodium, are held electronically within the open structure (porespace)—up to 38% void space.

The zeolite is present in the composition in an amount sufficient toincrease the flexural strength of the cementitious composition, whencured, as compared to the flexural strength of a neat cementitiouscomposition (the cementitious composition void of zeolite), when cured.Typically, the weight percent of zeolite in the composition is less thanor equal to 15 percent, preferably less than or equal to 10 percent.

The cementitious slurries of the invention may further containconventional cement slurry additives, such as suspending agents,dispersants, viscosifiers, suspending agents, defoamers and extendingagents. The amount of additive typically is dependent on the type ofhydraulically-active cementitious material used and desired density ofthe slurry. Such additives are typically present in an amount of from 0to about 5 pounds per barrel of mix water (hydraulically-activecementitious material and water), with about 1 to about 3 pounds perbarrel preferred.

Examples of retarders are lignin and sugar derivatives. Defloculents ordispersants to control fluidity include lignosulfonates, naphthalenesulfonates, phenol sulfonates, phosphates, phosphonates, sulfonatedstyrene maleic anhydride, sulfonated styrene, maleimide, polyacrylatesand polymethacrylates. Viscosity reducers include organic acids.

The cementitious slurry of the invention may further contain asuspending agent for maintaining the slurry with minimal separation ofthe cementitious material. Certain types of suspending agents of thetype used in the drilling mud industry can be used for the purposes ofthe present invention. Suitable suspending/thixotropic agents includewelan gum, xanthan gum, cellulose, polyanionic cellulose, xanthan gums,cellulose and derivatives such as carboxymethyl-hydroxyethyl-cellulose,guar and its derivatives, starch and polysaccharides, succinoglycan,polyethylene oxide, bentonite, attapulgite, mixed metal hydroxides,clays such as bentonite and attapulgite, mixed metal hydroxides, oil inwater emulsions created with paraffin oil and stabilized withethoxylated surfactants, poly (methyl vinyl ether/maleic anhydride)decadiene copolymer etc. Preferred as suspending agent are iotacarrageenan and poly (methyl vinyl ether/maleic anhydride) decadienecopolymer.

Mixing water containing the optional above-mentioned additives with thedry hydraulically-active cementitious materials produces the slurry. Asufficient amount of water, preferably fresh water, should be added tothe hydraulically-active cementitious material to form a liquid slurryof suitable consistency. The amount of water used in forming the cementslurry depends upon the type of hydraulic cement selected and the jobconditions at hand. The amount of water used can vary over a wide range,depending upon such factors as the required consistency of the slurryand upon the strength requirement for a particular job.

Depending upon the particular storable slurry, the amount of mixingwater in the slurry of the present invention typically ranges from about30 to 150 weight percent based upon the dry weight of cement andpreferably is in the range of about 35 to 90 weight percent. Forinstance, a slurry with Portland cement should have a density measuringin the range from about 11 to 17.5 lbm/gal and preferably in the rangeof about 14 to 17.5 lbm/gal, more preferably about 15-16.5 lbm/gal.Slurry densities for slag slurries of about 15 lbm/gal are preferable.

After the slurry is formed, the slurry is activated by the addition ofan activator and the slurry is then introduced into the well bore usingconventional methods so that the slurry fills the annular space betweenthe casing and the wall of the borehole.

Activators and activation methods as described in U.S. Pat. Nos.5,447,197 and 5,547,506 may be employed, including “over-activation.”Activators are typically added just prior to use of a storable cementslurry. Typical activators include, but are not limited to, solutions ofGroup IA and IIA hydroxides and carbonates, such as sodium hydroxide(caustic), potassium hydroxide, magnesium hydroxide, calcium hydroxide,sodium carbonate and calcium carbonate; Group IA halides, such as sodiumfluoride and KF; ammonium halides, such as ammonium fluoride andammonium bifluoride (ABF); sulfates, such as sodium sulfate; aluminates,such as sodium aluminate and potassium aluminate; carbonates, such asalkali carbonates, like sodium carbonate; phosphates, such as dibasicalkali phosphates (like dibasic potassium phosphate) and tribasic alkaliphosphates (like tribasic potassium phosphate); ammonium phosphates,such as tribasic ammonium phosphate and dibasic ammonium phosphate;silicates; and amines (such as triethanolamine (“TEA”), diethanolamine,etc.. Most typical activators are alkali silicates, such as sodiumsilicates. For slag slurries a sodium silicate “Crystal 120H”,Crosfield, Warrington, England, with a particular silica/soda ratio isespecially preferred. Sodium silicate (“Crystal 100S”, Crosfield) with adifferent silica/soda ratio is especially preferred for hydraulic cementand slagment slurries.

Further preferred suspending agents include soda ash, calcium oxide,magnesium oxide, calcium nitrate, calcium nitrite, zinc oxide, zinccarbonate, titanium carbonate and potassium hydroxide.

Upon addition of the activator, the slurry cement, typically being lessdense than the drilling fluid, is then introduced downhole into the wellbore. Typical concentrations of activator range from 0 to about 8gallons per barrel (“GPB”) of slurry, typically about 1 to about 3 GPBof slurry and are typically added with mix water.

Extra water is preferably added to the slurry during activation. Thisextra water may be fresh water, sea water or brine. The extra water maycontain activator and additional additives, for instance, potassiumchloride, dispersants, viscosifiers, liquid suspensions of weightingagents and chemical extending agents.

The activated slurry can be adjusted to the desired density for aparticular cementing application. The slurry density can be increased bythe addition of a liquid suspension of a weighting agent, such astrimanganese tetraoxide. Lower density slurries can be prepared byadding more water and modifying activator concentrations, if required.Thus, storable slurry “concentrates” can be made in advance and dilutedwhen activated. For example, a 12.5 lbm/gal Portland cement slurry canbe prepared by adding 2-3 gallons sodium silicate activator per bbl ofslurry and around 38 gallons of additional water per bbl to a storableslurry having the initial density around 15.8 to 16.5 lbm/gal.

Preferably, a pumpable slurry is formed with a measured density rangingfrom about 11 to about 20 lbm/gal, more preferably in the range of about14 to about 16 lbm/gal and most preferably about 15 lbm/gal. Althoughthe latter is a lower density than conventional “neat cement”, themechanical properties of the set cement are appropriate for wellcementing applications. Furthermore, the volume yield increases and therheology improves by this slight density reduction.

The activation step may be performed at a location different from thatof preparing the cementitious slurry. In other words, thehydraulically-active, cementitious slurry may be formulated at onelocation, transferred to a second location, activated at the secondlocation, and then pumped into the subterranean formation for cementing.

When cured, the flexural strength to compressive strength ratio (FS/CS)of the cement composition is typically between from about 0.29 to about0.80. For instance, the compressive strength of the cementitiouscomposition, when cured at 350° F. for 72 hours, is less than or equalto 4,500 psi, preferably less than or equal to 4,000 psi, as compared toapproximately 7375 psi produced by a neat cement (i.e., an identicalcement without the zeolite) mixed at the same density and tested underthe same conditions.

EXAMPLES

The following examples will illustrate the practice of the presentinvention in its preferred embodiments. A high temperature test was usedsince the cement, under such conditions, will reach its final mechanicalproperties within 72 hours. Other embodiments within the scope of theclaims herein will be apparent to one skilled in the art fromconsideration of the specification and practice of the invention asdisclosed herein. It is intended that the specification, together withthe example, be considered exemplary only, with the scope and spirit ofthe invention being indicated by the claims which follow.

All parts/percentages are given in terms of weight units except as mayotherwise be indicated.

The flexural strength data was generated on a Gilson Company “ModelHM-138” Cement Strength Tester. The test apparatus consists of analuminum frame containing the load transmission, machine controls, and aloading beam positioned at one side of the frames. The flexuralstrengths were determined using 1.575 by 1.575 by 6.3-in. cement prismsusing test methods outlined in ASTM C 348, “Standard Test Method forFlexural Strength of Hydraulic Cement Mortars”.

All compressive strength testing was conducted in accordance with APISpec 10, “Specification for Materials and Testing for Well Cements”,Jul. 1, 1990.

The zeolite used is commercially available as Doucil A24 from IneosSilicas and is a hydrated sodium calcium zeolite (clinoptilolite) withan average particle size of 1.1μ. This zeolite is resistant to breakdown under extreme pressures.

Examples 1-5

Storable cementitious slurries were made using Portland cement“Dyckerhoff Class G” (Dyckerhoff Zementwerke, Wiesbaden, Germany) as thecementitious material. The cement was mixed with about 35 percent S-8, a200-325 mesh silica, 0.5 percent CD-33 cement dispersant, aketone/acetone formaldehyde condensate, 0.1 percent gallons per sack(gps) of R-21L, a liquid set retarder, 0.01 gallons per cubic feet (gpc)of FP-6L defoamer, and from 0 to 10 by weight of cement (BWOC) zeolite.Comparative Example 1 contains no zeolite. S-8, CD-33, R-21L and FP-6Lare all products of BJ Services Company. The density of the cementitiousslurry was approximately 15.5 pounds per gallon (ppg) havingapproximately 47% water by weight of cement.

Table I, below, summarizes the flexural strength and compressivestrength testing at 350° F. after 72 hours of each of the formulations.TABLE I Ex. BWOC Compressive No. Zeolite Flexural Strength StrengthFS/CS Ratio 1 0.0 4000 1632.00 7375 0.22 2 2.5 4700 1918.37 7450 0.26 35.0 3700 1510.20 2875 0.53 4 7.5 3750 1530.61 4000 0.38 5 10.0 39501612.24 3875 0.42

As set forth in the results for Example 2, a high compressive strength(7450 psi) was maintained with an associated higher flexural strengthvalue (1918 psi). This characteristic is advantageous where zonalisolation across a high Young's Modulus formation is required. As theamount of zeolite contained in the slurry increases, the compressivestrengths fell, but the flexural strengths were similar to thoseproduced with the base cement without the zeolite producing andimprovement on the FS/CS ratio. As set forth in Example 2, addition ofzeolite above a concentration of 2.5% BWOC produced lower compressivestrength than Example 1, but had little appreciable effect on flexuralstrength. By adjusting the concentration of zeolite from 0 to 15%, thecompressive strength of the hardened cement is reduced whereas theflexural strength remains constant or increases.

A great advantage of the invention is the ability to produce variableflexural strength/compressive strength ratios (FS/CS) depending on thezeolite concentration without the necessity for a necessarily highcompressive strength. A high FS/CS ratio is advantageous in those wellbore conditions where a low Young's Modulus (soft) reservoir formationis present and a minimal Young's Modulus contrast between cement andformation is desired. Examples 3, 4, and 5 all demonstrate slurrieshaving a high FS/CS ratio, ideally suited for soft formation conditions.

From the foregoing, it will be observed that numerous variations andmodifications may be effected without departing from the true spirit andscope of the novel concepts of the invention.

1. A cementitious composition for cementing an oil or gas well,comprising: a hydraulically-active cementitious material; andsubstantially spherical zeolite wherein the flexural strength tocompressive strength ratio (FS/CS) of the cement composition, whencured, is between from about 0.29 to about 0.80.
 2. The cementitiouscomposition of claim 1, wherein the zeolite is porous.
 3. Thecementitious composition of claim 1, wherein the compressive strength ofthe cementitious composition, when cured, is equal to or less than thecompressive strength of a neat cementitious composition containing nozeolite, when cured.
 4. The cementitious composition of claim 3, whereinthe compressive strength of the cementitious composition, when cured, isat least 15% lower than the compressive strength of the neatcementitious composition containing no zeolite, when cured.
 5. Thecementitious composition of claim 4 12, wherein the compressive strengthof the cementitious composition, when cured, is at least 25% lower thanthe compressive strength of the neat cementitious composition.
 6. Thecementitious composition of claim 10, wherein the zeolite is selectedfrom natrolite, heulandite, analcime, chabazite, stilbite, andclinoptilolite.
 7. The cementitious composition of claim 6, wherein thezeolite is clinoptilolite.
 8. The cementitious composition of claim 10,wherein the weight percent of zeolite in the composition is less than orequal to 15 percent.
 9. The cementitious composition of claim 8, whereinthe weight percent of zeolite in the composition is less than or equalto 10 percent.
 10. A cementitious composition for cementing an oil orgas well, comprising: a hydraulically-active cementitious material; andsubstantially spherical zeolite wherein the compressive strength of thecomposition, when cured, is equal to or less than the compressivestrength of a neat cementitious composition not containing the zeolite.11. The cementitious composition of claim 16, wherein the compressivestrength of the composition, when cured, is equal to or less than thecompressive strength of a neat cementitious composition.
 12. Thecementitious composition of claim 10, wherein the compressive strengthof the composition, when cured, is at least 15% lower than thecompressive strength of the neat cementitious composition. 13.(canceled)
 14. (canceled)
 15. (canceled)
 16. A cementitious slurry forcementing an oil or gas well, comprising: a hydraulically-activecementitious material; substantially spherical zeolite; and waterwherein the zeolite is present in the slurry in an amount sufficient toincrease the flexural strength of the slurry, when cured, as compared tothe flexural strength of a cementitious slurry, when cured, which doesnot contain zeolite.
 17. The cementitious slurry of claim 16, whereinthe amount of zeolite in the slurry is less than about 15 weightpercent.
 18. The cementitious slurry of claim 16, wherein the flexuralstrength to compressive strength ratio (FS/CS) of the slurry, whencured, is between from about 0.29 to about 0.80.
 19. The cementitiousslurry of claim 17, wherein the amount of zeolite in the slurry is lessthan or equal to about 10 weight percent.
 20. The cementitious slurry ofclaim 16, wherein the zeolite is selected from natrolite, heulandite,analcime, chabazite, stilbite, and clinoptilolite.
 21. The cementitiousslurry of claim 20, wherein the zeolite is clinoptilolite. 22.(canceled)
 23. (canceled)
 24. (canceled)
 25. A cementitious slurrycomprising a hydraulically-active cementitious material, substantiallyspherical zeolite and water wherein, when cured, the flexural strengthand tensile strength of the cementitious slurry is greater than theflexural strength and tensile strength of a neat cementitious slurrycontaining no zeolite when cured.
 26. The cementitious slurry of claim25, wherein the zeolite is selected from the group consisting ofnatrolite, heulandite, analcime, chabazite, stilbite and clinoptilolite.